1. Field of the Invention
Exemplary embodiments of the present invention generally relate to an image forming method and apparatus for effectively charging an image carrier, and more particularly to an image forming apparatus that can uniformly charge a surface of an image carrier while contacting a surface of a charging member to which a charge bias is applied, and an image forming method used in the above-described image forming apparatus.
2. Discussion of the Related Art
When producing copies of an original document, related-art electrophotographic image forming apparatuses cause a charging unit to uniformly charge a surface of an image carrier, an optical writing unit to irradiate the surface of the image carrier to form an electrostatic latent image on the surface of the image carrier, and a developing unit to develop the electrostatic latent image into a visible toner image. The visible toner image is transferred onto a recording medium such as a transfer sheet directly from the image carrier or onto an intermediate transfer member before being transferred onto a recording medium.
In a known charging unit, a charging member, such as a charge roller and a charging brush roller, contacts the image carrier to form a charge nip and apply a charge bias to the charge nip, thereby uniformly charging the surface of the image carrier.
In the related-art image forming apparatuses including such charging unit, residual toner may remain on the surface of the image carrier after the transfer operation of the toner image. As the image carrier rotates, the residual toner remaining on the surface of the image carrier may be conveyed to the charge nip and adhere to the charging member. As the amount of residual toner adhering to the charging member continues to increase, defective charging occurs in a local region or local regions on the surface of the image carrier, resulting in deterioration of image quality.
To avoid the above-described drawback, some related-art image forming apparatuses have a configuration in which a surface of a charge roller serving as a charging member travels in a direction opposite to a surface of an image carrier at a charge nip that is a contact portion of the charge roller and the image carrier, so that the surface of the image carrier can be uniformly charged.
The related-art image forming apparatuses having the above-described configuration cause the charge roller to rotate after a print job and/or before an image forming operation at a speed greater than a speed thereof generated during the image forming operation. By speeding up the rotation of the charge roller as described above, the residual toner remaining on the charge roller can effectively be discharged to the image carrier, that is, the amount of residual toner discharged can be increased.
A detailed description is now given of the enhancement of toner discharge efficiency or efficiency of discharging residual toner remaining on the charge roller in a related-art image forming apparatus having the above-described configuration.
FIG. 1 shows a schematic configuration of an image forming part of a related-art image forming apparatus 200.
The related-art image forming apparatus 200 includes a charge roller 201, a photoconductor 202, and a developing unit 205. The charge roller 201 and the photoconductor 202 form a charge nip therebetween.
As indicated by arrows shown in FIG. 1, the surface of the charge roller 201 travels in a direction opposite to a direction of travel of the surface of the photoconductor 202 at the charge nip.
A point P11 located at the right end of the charge nip in FIG. 1 corresponds to an entrance or start point of the charge nip of the charge roller 201. Toner particles T adhering to the surface of the charge roller 201 enter the charge nip via the point P11 in FIG. 1. At this time, the photoconductor 202 traveling in the opposite direction to the charge roller 201 at the charge nip exerts a force of removing or scraping the toner particles T. Therefore, the toner particles T are discharged from the charge roller 202 at the point P11. The discharged toner T is then conveyed along the surface of the photoconductor 202 and to the developing unit 205, without entering the charge nip.
In the image forming apparatus 200 having the above-described configuration, when the rotation speed of the charge roller 201 increases, a difference between the linear velocity of the charge roller 201 and the linear velocity of the photoconductor 202 may increase, thereby increasing a removing force of the toner particles T at the point P11.
According to the above-described operations, it is believed that the efficiency of discharge of the toner particles T from the charge roller 201 can be enhanced.
However, when the above-described configuration or a first configuration in which the surface of the charge roller 201 and the surface of the photoconductor 202 travel in the opposite directions at the charge nip is compared with a second configuration in which the surface of the charge roller 201 and the surface of the photoconductor 202 travel in the same direction at the charge nip, the image forming apparatus 200 with the first configuration may require a greater amount of a driving torque generated by each driving source for the charge roller 201 and the photoconductor 202. Therefore, the driving source for the first configuration may need to be larger than the driving source for the second configuration, and such a large-sized driving source can increase costs.
Consequently, to reduce such costs, it would be preferable to use the second configuration and have the surface of the charge roller 201 and the surface of the photoconductor 202 to travel in the same direction at the charge nip. However, the inventors of the present invention have conducted tests and found that the second configuration cannot efficiently increase the toner discharge efficiency of the charge roller 201 even when the rotation speed of the charge roller 201 is increased. That is, it is believed that, when the surface of the charge roller 201 travels in the same direction as the surface of the photoconductor 202, the surface of the photoconductor 202 cannot apply the toner removing force to the toner T remaining on the surface of the charge roller 201.